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FORGED STEEL, CAST ALUMINUM AND CAST IRON IN SAFETY CRITICAL
AUTOMOTIVE COMPONENTS
George F. MochnalDirector of Research and Education
Forging Industry AssociationForging Industry Educational and Research Foundation
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DRIVERS TO CHANGE
Need for significant reduction in energy usageNeed for significant reduction in energy usageAdvances in fundamental understanding of Advances in fundamental understanding of interactions between materials and processesinteractions between materials and processesAccess to information Access to information -- integrationintegrationEnvironmental driversEnvironmental driversImproved process knowledge based on Improved process knowledge based on scientific information and improved sensing scientific information and improved sensing and controland controlCompetitive pressure for sustained Competitive pressure for sustained productivity improvementsproductivity improvementsEmergence of netEmergence of net--shape processes and shape processes and engineered solutionsengineered solutions
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RESEARCH PARTNERS
Department of Defense (DOD)Forging Defense Manufacturing Consortium
Department of Commerce (DOC) National Institute of Standards and Technology (NIST)
Department of Energy (DOE)Office of Industrial Technologies - Industries of the Future
National LaboratoriesOak Ridge National Laboratory, Sandia National Laboratory, Argonne National Laboratory
Universities
Joint industry Alliances (JIA)
Collaborative Industry projects
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FORGING PROGRAMS
Precision Forging Consortium Precision Forging Consortium -- extend tool lifeextend tool life
Innovative die material & lubrication strategies Innovative die material & lubrication strategies
Center of Excellence in Forging Technology Center of Excellence in Forging Technology –– Ohio State Ohio State UniversityUniversity
Fast nonFast non--contact dimensional and surface finish contact dimensional and surface finish inspection of forgings at elevated temperatures.inspection of forgings at elevated temperatures.
Effects of induction processing on the mechanical Effects of induction processing on the mechanical properties of steels with controlled microstructuresproperties of steels with controlled microstructures
Heating response of different starting microstructures Heating response of different starting microstructures and their relationships with important forging process and their relationships with important forging process parameters parameters
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Heat Treating Society
Steel, Forging and Heat Treating Industry Pact
Joint Industry Alliance
(JIA)
JOINT INDUSTRY ALLIANCE(JIA)
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Forging Industry Educational & Forging Industry Educational & Research Foundation (FIERF)Research Foundation (FIERF)
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FORGED STEEL, CAST ALUMINUM AND CAST IRON IN SAFETY-CRITICAL
AUTOMOTIVE COMPONENTS
Mehrdad Zoroufi, Graduate Assistant and
Ali Fatemi, ProfessorThe University of Toledo
Funded by:
and
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OVERALL OBJECTIVES
• Evaluate and compare fatigue performance of forged steel components with other process technologies.
• Steering knuckle chosen as example part:– Forged Steel– Cast Aluminum– Cast Iron
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PROJECT OUTLINE
• Literature Survey– Forging vs. Competing Manufacturing Processes– Potentials for Improving Competitiveness– Durability & Optimization of Steering Knuckle
• Experimental Work– Specimen Testing
• Forged steel SAE 11V37• Cast aluminum ASTM A356-T6• Cast iron ASTM A536 Grade 65-45-12
– Component Testing on forged steel and cast aluminum knuckles
• Determining test configuration• Manufacturing fixtures• Load-control fatigue testing
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PROJECT OUTLINE
• Analytical Evaluations– Digitizing steering knuckle Geometry
• Forged steel • Cast aluminum • Cast iron
– Stress/Strain (FEA) Analysis• Forged steel knuckle• Cast aluminum knuckle • Cast iron knuckle
– Durability (Fatigue) Analysis – Optimization Analysis
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PUBLICATIONS AND PRESENTATIONS
• Fatigue Performance Evaluation of Forged vs. Competing Process Technologies: A Comparative Study, 24th FIA Conference, Cleveland, OH, October 2002.
• A Comparative Study of Forged Steel 11V37, Cast Aluminum A356-T6, and Cast Iron 65-45-12; Monotonic Properties, Cyclic Deformation, and Fatigue Behavior, Technical Report to FIERF and AISI (available through www.forging.org), March 2003.
• Forged Steel, Cast Aluminum and Cast Iron in Safety Critical Automotive Components, Great Designs in Steel Seminar 2004, Livonia, MI (February 18th 2:30 PM).
• Fatigue Life Comparison of Competing Manufacturing Processes: A Study of Steering Knuckle, (SAE Technical paper 2004-01-0628) SAE World Congress 2004, Fatigue Research & Applications Session, Detroit, MI (March 9th 11:30 AM).
• Durability Comparison and Life Predictions of Competing Manufacturing Processes: An Experimental Study of Steering Knuckle, 25th FIA Conference, Detroit, MI (April 20th 1:50 PM).
• Experimental and Analytical Durability Assessment of Vehicle Steering Knuckle, Jr. Auto. Eng’g, in review.
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Forged steel
SAE Grade 11V37
Steering knuckle of rear suspension of a
4-cylinder sedan weighing 2.5 kg
Cast iron
ASTM A536 Grade 65-45-12
Steering knuckle of front suspension of a 4-cylinder
sedan weighing 4.7 kg
Cast aluminum
ASTM A356-T6
Steering knuckle of front suspension of a
6-cylinder minivan weighing 2.4 kg
STEERING KNUCKLES
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STEERING KNUCKLE WITHINTHE SUSPENSION SYSTEM
Knuckle body connects to strut, front and rear lateral links, and tension strut/chassis bracket
Spindle attaches to wheel hub & bearing assembly
Steering, braking, cornering, hitting pot holes cause cyclic loads
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Specimen TestingStrain-controlled tensile tests
Strain-controlled fatigue tests
Based on ASTM standards and recommended practices
Flat specimens from forged steel, cast aluminum, and cast iron steering knuckles
Instron closed-loop servo-controlled hydraulic axial load
frame and digital servo controller are utilized for the tests.
SPECIMEN TESTING
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2.54
76
6.985
9.53
2.5
24.5R1
2.7
Flat plate specimen configuration and dimensions
according to ASTM Standard E606
(All dimensions in mm)
SPECIMEN GEOMETRY
Generating specimens
Forged steel knuckle:
Cast aluminum and cast iron knuckles: hub and arms
B
AC
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COMPARISON OF MATERIAL MECHANICAL PROPERTIES
Forged Steel Cast Aluminum Cast Iron
11V37 A356-T6 65-45-12YS (MPa) 556 232 300YS Ratio 1 0.41 0.54UTS (MPa) 821 302 471UTS Ratio 1 0.37 0.57%RA 37 10 25%RA Ratio 1 0.27 0.68YS’ (MPa) 541 291 407YS’ Ratio 1 0.54 0.75Sf (at Nf=106) 352 122 253Sf Ratio 1 0.35 0.72
* base of comparison is 11V37.
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0
100
200
300
400
500
600
700
0.0% 0.2% 0.4% 0.6% 0.8% 1.0%
True Strain (%)
True
Str
ess
(MPa
)
Forged Steel 11V37Cast Iron 65-45-12Cast Aluminum A356-T6
Monotonic
Cyclic
Monotonic
Cyclic
Monotonic
Cyclic
MATERIAL CYCLIC & MONOTONIC STRESS-STRAIN BEHAVIOR
ε ε εσ σ
= + = +⎛⎝⎜
⎞⎠⎟e p
n
E K
1
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MATERIALSTRESS-LIFE COMPARISON
Better overall S-N fatigue resistance of the forged steel
Long-life fatigue strengths of cast aluminum 35% and cast iron 72% of forged steel
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Fx
Forces and Moments on the Test Road (Lee, 1995)
Fy
Fz
Mz
Mx
Wheel Forces and Moments (Lee, 1995)
Occasional OL’s and UL’s cause inelastic deformation
EFFECT OF OVERLOADS
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0.001%
0.010%
0.100%
1.000%
1E+2 1E+3 1E+4 1E+5 1E+6
Reversals to Failure, 2Nf
True
Pla
stic
Str
ain
Am
plitu
de (%
)
Forged Steel 11V37Cast Iron 65-45-12Cast Aluminum A356-T6
MATERIALPLASTIC STRAIN COMPARISON
Overloads are common for suspension components such as steering knuckle
Cyclic plasticityan index for tolerance against overloads
Higher capacity of forged steel for cyclic plastic deformation
Better low-cycle fatigue behavior
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0.1
1.0
10.0
1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8
Reversals to Failure, 2Nf
ε aσ
a (M
Pa)
Forged Steel 11V37Cast Iron 65-45-12Cast Aluminum A356-T6
MATERIALNEUBER CURVE COMPARISON
Neuber Plot:is useful when
analyzing component geometries with stress concentrations
Notch root fatigue behavior is a function of both local stress and strain
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0.10%
1.00%
1E+3 1E+4 1E+5 1E+6 1E+7
Reversals to Failure, 2Nf
True
Str
ain
Am
plitu
de, Δ
ε/2,
%
A
C
B
B
C
B
ALongitudinal direction experiencing the highest level of stress, exhibits best fatigue strength.
DIRECTIONAL FATIGUE STRENGTHOF THE FS KNUCKLE
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FORGED STEEL KNUCKLE
BOUNDARY CONDITIONSFOR STRESS (FEA) ANALYSIS
CAST ALUMINUM KNUCKLE
CAST IRON KNUCKLE
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FORGED STEEL KNUCKLE
MESH DETAILS CAST
ALUMINUM KNUCKLE
CAST IRON KNUCKLE
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Contours of von Mises stress showing the critical locations for:
forged steel knuckle (left): spindle 1st step fillet
cast aluminum knuckle (middle): hub bolt holes
cast iron knuckle (right): strut arm root and hub bolt hole
STRESS ANALYSIS RESULTSCRITICAL LOCATIONS
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FORGED STEEL KNUCKLE TEST SETUP
• Spindle was fixed by a two-piece block
• Threaded rods tightened the block to the spindle
• A pair of L-shaped moment arms transferred the load from the testing machine loading actuator to the spindle blocks in the form of a bending load
• Strut and suspension connections on the knuckle body were fixed to the bench using round and square blocks
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CAST ALUMINUM KNUCKLE TEST SETUP
• Strut attachment of the arm was connected from both sides to a pair of moment arms.
• Moment armstransferred the bending load from the loading actuator to the knuckle.
• Four hub bolt holeswere fixed to the bench.
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To verify testing configuration and FEA results
Measured and predicted strain values at 2.2 kN static load.
All measurements after a few cycles
Locations of the gages are shown with arrows.
FEA and measured strains are relatively close
MEASURED & PREDICTED STRAINS
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0.5
1.5
0 0.2 0.4 0.6 0.8 1
Normalized Number of Cycles, N/Nf
Dis
plac
emen
t Am
plitu
de (m
m) Forged Steel Knuckle
Cast Aluminum Knuckle
crack nucleates
COMPONENT FAILURE CRITERIA
To record macro-crack nucleation, growth, and fracture stages A marked Δdincrease considered as crack nucleation point, and a sudden increase as fracture.For cast aluminum knuckle, crack growth portion was significant.
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FRACTURE SURFACES
crack nucleation
crack propagation
fracture
crack nucleation
crack propagation
fracture
FORGED STEEL
KNUCKLE
CAST ALUMINUM KNUCKLE
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COMPONENT TEST RESULTS& STRESS-LIFE COMPARISONS
Forged steel knuckle gives about two orders of magnitudelonger life than cast aluminum knuckleApparent fatigue limitfor forged steel knuckle, but not cast aluminum knuckleHighest load levels represent overload conditions for suspension components, such as steering knuckle
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100
1000
1E+3 1E+4 1E+5 1E+6 1E+7Cycles to Failure, Nf
Stre
ss A
mpl
itude
(MPa
)
Forged Steel Knuckle - Test DataCast Aluminum Knuckle -Test DataForged Steel Knuckle - PredictionCast Iron Knuckle - PredictionCast Aluminum Knuckle - Prediction
COMPONENT STRESS-LIFE PREDICTIONS
Predictions based on the S-N approach are conservative for both forged steel and cast aluminum knuckles. This is partly due to the conservative nature of the modified Goodman equation.
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COMPONENT STRAIN-LIFE PREDICTIONS
• Predictions based on the ε-N approach are relatively close to experimental lives for forged steel and cast aluminum knuckle.
• Life Predictions:Forged steel 10X cast iron Forged steel 30X cast aluminum
Forging Influencing Parameters
Influenced Parameters
Influenced Mechanical Properties
Forging Method
&Sequence
Workpiece Material
Workpiece Geometry
Die & Tool Design
Forging Temperature
Workpiece Temperature
Contact Time
Loading Rate
Monotonic Behavior
Cyclic Behavior & Fatigue
Defects • Porosity • Inclusions
Surface Finish
• Roughness • Scale
Residual Stresses
Fracture Behavior & Damage Tolerance
Impact & Dynamic Behavior • CVN
Energy
• •
ε Effect
Microstructure• Grain Type
and Size • Grain Flow
Other Behavior & Properties • Wear
Resistance • Fretting
Behavior • Corrosion
Resistance • Creep
Behavior
Cyclic Strength
σ’f , b, Sf, S’y
Stiffness E
Tensile Strength Su , Sy , σf
Ductility %EL, %RA,
Tensile Toughness &
Resilience
Cyclic Ductility
ε’f , c
Cyclic Strain Hardening
K’, n’
Tensile Strain
Hardening K, n
Hardness
Fracture Toughness K1c, Kc, J1c
Crack Growth
Resistance Α, m, ΔKth
Friction and Lubrication
FORGING & FATIGUE
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DESIGN VARIABLES(can change)
• Material– Replacing by microalloyed
steel– etc.
• Manufacturing process– Forging steps– Machining– Grinding– etc.
• Part processing– Heat treatment– Surface treatment– Inducing compressive residual
stress– Induction hardening– Rolling– etc.
CONSTRAINTS(cannot change)
• Equivalent or better fatigue strength
• Geometry:– Strut mounting bolt-hole size
and location– Suspension connection bolt-
hole size and location– Spindle diameters– Spindle length
DEFINITION OF OPTIMIZATION PROBLEM
OBJECTIVE FUNCTION
Weight of FS knuckle
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• From tensile tests, cast aluminum and cast iron reached 37% and 57% of forged steel UTS, and percent elongation were 24% and 48% of forged steel, respectively.
• Cyclic yield strength of cast aluminum and cast iron were found to be 54% and 75%, while the cyclic strain hardening exponent were 46% and 55% of forged steel, respectively. These indicate higher resistance of the forged steel to cyclic plastic deformation.
• Long-life material fatigue strengths of cast aluminum and cast iron are only 35% and 72% of forged steel, respectively.
• Comparisons of strain-life fatigue behavior indicates forged steel provides more than an order of magnitude longer life than the cast iron, and more than a factor of 3 longer life than castaluminum.
CONCLUSIONS
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• Neuber plots of the three materials, that is useful when analyzing component geometries with stress concentrations, show a factor of six longer life for forged steel than that of cast iron and three orders of magnitude longer life than that of cast aluminum.
• Differences between measured and FEA-predicted strains were found to be reasonable for the complex knuckle geometries considered. FEA-predicted and observed failure locations were identical.
• Crack growth life was found to be a significant portion of cast aluminum knuckle fatigue life, while it was not significant for forged steel knuckle fatigue life.
CONCLUSIONS
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• Component test results showed forged steel knuckle to have about two orders of magnitude longer life than cast aluminum knuckle, for the same stress amplitude level. This occurred at both short as well as long lives.
• S-N life predictions for the components were overly conservative, whereas strain-life predictions were relatively close to component experimental results.
• Comparison of the strain-life prediction curves of the components demonstrated that forged steel knuckle offers more than an order of magnitude longer life than cast iron knuckle.
CONCLUSIONS
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